linux-kernel - Re: 2.6.33-rc1: hrtimers and tickless broken on m68knommu.

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Date:	Fri, 18 Dec 2009 21:06:18 -0800
From:	Steven King <sfking@...dc.com>
To:	john stultz <johnstul@...ibm.com>
Cc:	linux-kernel@...r.kernel.org, Greg Ungerer <gerg@...pgear.com>
Subject: Re: 2.6.33-rc1: hrtimers and tickless broken on m68knommu.

On Friday 18 December 2009 08:04:23 john stultz wrote:
> On Fri, 2009-12-18 at 19:20 -0800, Steven King wrote:
> > On Friday 18 December 2009 06:44:44 john stultz wrote:
> > > On Fri, Dec 18, 2009 at 6:13 PM, Steven King <sfking@...dc.com> wrote:
> > > > Attach is the .config; it works on v2.6.32 but fails to boot on
> > > > .33-rc1; but if I deselect hrtimers && tickless then it works.
> > >
> > > Sorry for the dup, forgot to cc lkml on my reply.
> > >
> > > Fails to boot all together? Or does it hang at some point in the dmesg
> > > that you can point out?
> >
> > fails to boot all together; nothing on the serial console.
> >
> > > Could you run the following so we can narrow down which clocksource
> > > your using? cat
> > > /sys/devices/system/clocksource/clocksource0/current_clocksource cat
> > > /sys/devices/system/clocksource/clocksource0/available_clocksource
> > >
> > > Then with the kernel that doesn't boot, go through the clocksources
> > > listed in available_clocksources and try booting w/
> > > "clocksource=<clock name>" and see if the behavior changes.
> >
> > on the working .32 kernel:
> >
> > # cat /sys/devices/system/clocksource/clocksource0/current_clocksource
> > pit
> > # cat /sys/devices/system/clocksource/clocksource0/available_clocksource
> > pit
> >
> > just to be sure, I tried clocksource=pit on the .33-rc1 kernel.  It didnt
> > make any difference.
>
> Hrmm.. So looking at the code in arch/m68knommu/platform/coldfire/pit.c,
> I'm a little confused on how this got marked as a continuous clocksource
> (CLOCK_SOURCE_IS_CONTINUOUS), especially as it seems it couldn't handle
> skipping an interrupt.
>
> That said, I'm not sure how it worked in 2.6.32, as its been that way
> for awhile it seems.  Maybe my assumptions on how the PIT works is wrong
> (or just biased in how it works on x86)?
>
> Greg, could you clarify how the PIT can be used as a clocksource if its
> also being used in oneshot mode?
>
> Steven, I assume the patch below avoids the issue (by disabling highres
> timers and nohz)?

Yes.

I suspect it wasnt working correctly on earlier kernels, we just got away with 
it; I had recently added ntpclient to this target but the time reported by 
date was always off by some odd amount, I had assume that it was a busybox or 
ntpclient issue but hadnt gotten around to tracking it down.  With your patch 
(or, as I just now verified, on .32 without no_hz and hrtimers) the system 
time is now correct.  I probably never would have made the connection.

Thank you John!

-- 
Steven King -- sfking at fdwdc dot com
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